The present invention relates to a method of utilizing expanded material to produce moulded articles with portions of different density thus increasing the versatility of the article produced.
According to the present invention a process for moulding articles of fibre reinforced plastics material includes pre-heating a sheet of consolidated thermoplastics material which has dispersed therein from 20% to 70% by weight of reinforcing fibres which have a high modulus of elasticity (as herein defined) and between about 7 and about 50 millimeters long to cause the sheet to expand due to relief of the stresses in the reinforcing fibres, placing the expanded sheet in a compression mould and closing the mould to form an article of predetermined shape having portions of different density.
A high modulus of elasticity is to be taken as meaning a modulus of elasticity substantially higher than that of the sheet. Fibres falling into this category include glass, carbon and ceramic fibres such as the aramid fibres sold under the trade names KEVLAR AND NOMEX and will generally include any fibre having a modulus higher than 10,000 Mega Pascals.
The difference in density can be achieved by forming the sheet material in different thicknesses and/or the process could include shaping the forming tool mould to compress the sheet to fully compact it in the area of greater density.
The portions of greater density of the moulded article thus produced can be utilised to locate an insert, for example a bush or clip, and thus the invention also includes incorporating an insert in the portion of greater density during the moulding process.
Preferably the thermoplastic material has at least 30% by weight glass fibres and thus it may have 50% by weight glass fibres.
Preferably, the fibres are in the form of single discrete fibres. Thus, where glass fibres are used, and are received in the form of chopped strand bundles, the bundles are broken down into single fibres before the structure is formed.
The sheet may be formed of two more more layers of plastics material of different compositions which are laminated together in the moulding process. Thus three or more layers may be incorporated, the outer layers having a lower fibre content by weight than the inner layer or layers.
The thermoplastics materials may for example be of polyethylene, polypropylene, polystyrene, acrilonitrystyrenebutadiene, polyethyleneterephthalate or polyvinylchloride, both plasticised and unplasticised. Other suitable thermoplastics include polyphenylene ether or polycarbonates or polyestercarbonates or thermoplastic polymers or polyetherimides or acrylonitrile--burylacrylate--styrene polymers or amorphous nylon or polyarylene ether ketone or alloys and blends of these materials with each other or other polymeric materials.